Image quality, particularly from 70-200, but good all the way to 300. IS is excellent. Close focusing distance. Relatively light weight and compact size for easy portability.

Cons:

Would benefit greatly from a focus limiter, and from ring USM/full time manual focus. Rotating front element doesn't bother me, but will inconvenience some. "Zoom creep" can be a problem. See my solution :)

This is a superb performer for its class of lens. I am particularly impressed with the image quality wide open from 70 to 240mm or so. Much better results (for me) than I got with Sigma 70-200 f2.8, mostly attributable to the IS. This lens does focus closer than the Sigma, which is an advantage.